Nocturnal Foraging by Red-Legged Kittiwakes, a Surface Feeding Seabird That Relies on Deep Water Prey During Reproduction

Abstract

Narrow foraging specialization may increase the vulnerability of marine predators to climate change. The red-legged kittiwake (Rissa brevirostris) is endemic to the Bering Sea and has experienced drastic population fluctuations in recent decades, presumably due to climate-driven changes in food resources. Red-legged kittiwakes are presumed to be a nocturnal surface-foraging seabird that feed almost entirely on deep water Myctophidae fishes. However, there is little empirical evidence confirming their nocturnal foraging activity during the breeding season. This study investigated the foraging behavior of red-legged kittiwakes by combining GPS tracking, accelerometry, and dietary analyses at the world's largest breeding colony of red-legged kittiwakes on St. George I. GPS tracking of 5 individuals revealed that 82.5% of non-flight behavior (including foraging and resting) occurred over the ocean basin (bottom depth >1,000 m). Acceleration data from 4 birds showed three types of behaviors during foraging trips: (1) flight, characterized by regular wing flapping, (2) resting on water, characterized by non-active behavior, and (3) foraging, when wing flapping was irregular. The proportions of both foraging and resting behaviors were higher at night (14.1 ± 7.1% and 20.8 ± 14.3%) compared to those during the day (6.5 ± 3.0% and 1.7 ± 2.7%). The mean duration of foraging (2.4 ± 2.9 min) was shorter than that of flight between prey patches (24.2 ± 53.1 min). Dietary analyses confirmed myctophids as the dominant prey (100% by occurrence and 98.4 ± 2.4% by wet-weight). Although the sample size was limited, these results suggest that breeding red-legged kittiwakes concentrated their foraging on myctophids available at the surface during nighttime in deep water regions. We propose that the diel patterns and ephemeral nature of their foraging activity reflected the availability of myctophids. Such foraging specialization may exacerbate the vulnerability of red-legged kittiwakes to climate change in the Bering Sea.

Fig 1. Frequency distribution of ground speed (m/s) of red-legged kittiwakes as obtained by GPS loggers.

The black arrow (3 m/s) shows a cut-off value discriminating flight and non-flight behaviors.

Fig 2. Locations of different types of behavior of red-legged kittiwakes as revealed by GPS loggers in relation to bathymetry.

The blue dots represent flight behavior, the open light blue circles represent non-flight behavior (including foraging and resting on water) that occurred in ocean basin (where bottom depth >1,000 m), the open yellow circles represent non-flight behavior that occurred on the shelf break (bottom depth: 200 to 1000 m), and the open red circles represent non-flight behavior that occurred on the shelf (bottom depth: 0 to 200 m). The open green circles represent bathing behavior which occurred on land.

Fig 3. Behavioral patterns of red-legged kittiwakes as revealed by accelerometers.

Panel A shows a time series of depth, heave acceleration, body angle, dominant amplitude of the heave acceleration, dominant cycle of the heave acceleration, and behavioral categories determined by accelerometry (the colors correspond to each behavioral category). Panel B shows enlarged records from panel A.

Fig 4. Diel behavioral patterns of red-legged kittiwakes.

The left panels (A, B, and C) represent the data from GPS loggers, and the right panels (D, E, F) represent the data from accelerometry. Sunset, sunrise, twilight and dark night times are shown at the top. The top panels (A and D) show data available for each hour of a day, the middle panels (B and E) show % data occurrence, and the bottom panels (C and F) show patterns of complete foraging trips. In the bottom panels, solid lines represent the range when the data were obtained for each trip. The trip ID is shown by the vertical axis. The closed and open circles represent the timing of the start and the end of the trips, respectively.